The present invention relates to tank pressure control systems, and in particular to an apparatus for controlling flow of fuel vapor and liquid fuel through an aperture in a fuel tank. More particularly, the present invention relates to a tank venting control assembly having a float valve that snap closes quickly and automatically to prevent discharge of fuel vapor and liquid fuel from the fuel tank during refueling, agitation of the tank, or vehicle rollover.
Fuel vapor can be created in the fuel tank by temperature differences between the fuel tank and liquid fuel from a fuel pump, as well as by sloshing and agitation of the fuel tank during normal vehicle operation. The pressure buildup resulting from the creation of new fuel vapors must be relieved properly. For this reason, many vehicle fuel systems are equipped with tank venting control assemblies capable of discharging a relatively large amount of fuel vapor in response to the development of high pressure conditions in the fuel tank. See, for example, U.S. Pat. No. 5,234,013 to Roetker et al. Typically tank venting control assemblies also include float valves to provide rollover closure and aid overfill shutoff functions. For a description of such a float valve, see U.S. Pat. No. 5,065,782 to Szlaga.
It has long been desirable to maximize the sensitivity that can be achieved with tank venting control assemblies while at the same time minimizing manufacturing costs and assembly difficulties. It would be desirable to provide a tank venting control assembly sensitive enough to begin discharging fuel vapor from the fuel tank in response to pressure differentials as low as about 1-2 inches (2.5-5.1 cm) of water. In fact, a valve design meeting these requirements would closely simulate an open orifice. In any event, meeting the sensitivity requirement calls for a valve which can develop a relatively high actuation force in response to relatively low pressure differentials, long the focus of engineering efforts in this area.
Some tank venting control assemblies have relied upon the use of a small amount of fuel vapor from the fuel tank to provide a pressure signal to actuate the venting assembly, allowing discharge of large volumes of fuel vapor from the fuel tank. Such systems are disclosed, for example, in U.S. Pat. Nos. 4,953,583 and 5,065,782. However, it has proven difficult to design such systems to achieve the desired sensitivity without compromising manufacturability.
In addition to providing for adequate fuel vapor discharge from the fuel tank during high tank pressure conditions, well-designed tank pressure control assemblies must be capable of responding to the reduction of pressure in the fuel tank to below a predetermined level. These tank vacuum conditions are usually relieved by introducing ambient air to the fuel tank to bring the fuel vapor pressure in the fuel tank back to approximately atmospheric pressure. However, it has long been problematic to provide adequate tank vacuum relief functions without complicating the tank venting control assembly unnecessarily.
Further, it has long been appreciated that venting control assemblies should provide means for preventing liquid fuel leakage during tilting or rollover of the vehicle (see e.g., U.S. Pat. Nos. 4,351,350; 4,655,328; and 4,760,858). However, venting control assemblies allowing discharge of high mass flow rates of fuel vapor should also provide means for reducing discharge of liquid fuel even when the fuel tank is in its normal upright position.
Tank pressure control systems typically include a float valve therein in order to control this discharge of liquid fuel as shown, for example, in U.S. Pat. No. 5,065,782. It is well understood that said float valve becomes buoyant once the buoyant force (the product of a cross-sectional area of the float valve, submerged height of the float valve, and specific gravity of fluid surrounding the float valve) overcomes the weight of the float valve. Often these float valves are designed to include a bottom end having either a structure with a constant cross-sectional area or a cross-sectional area that gradually increases as the fluid level rises within the fuel tank. As rising fluid meets either of these types of bottom ends, the fluid surrounds the outer periphery of the valve and causes the submerged height of the valve to increase while the perceived cross-sectional area remains constant or increases gradually.
Under circumstances such as these, the only variable within the buoyancy equation undergoing significant change is the submerged height of the valve. Thus, float valves that behave as substantially uniform cross-sectional structures, rise at a rate proportional to the rate of the rising fluid. This proportional rise could however lead to premature closure problems under high fluid-flow conditions within the fuel tank. To overcome premature closure at high fluid-flow conditions, manufacturers often position the float valve within the housing from about 3.18 mm to 6.35 mm away from the desired closed position. However, increases in travel distance to the closed position can result in a reduction in pressure sensitivity of the valve. What is needed is a venting control assembly that maximizes the sensitivity of a float valve to pre-determined closing conditions while at the same time reducing the incidence of premature closure of the valve.
According to the present invention, an apparatus is provided for controlling venting of fuel vapor through an aperture in a vehicle fuel tank and for introducing air into the fuel tank in response to the development of vacuum conditions therein. The apparatus includes a housing which is mounted in the aperture of the fuel tank and has an upper chamber, a lower chamber, and a float valve disposed in the lower chamber of the housing. The housing is further formed to include an outlet and defines a vent path for fuel vapor between the fuel tank and the outlet.
The float valve is movable within the lower chamber of the housing between a closed position blocking flow of fuel vapor and liquid fuel through the vent path and an open venting position. The float valve has a vertical axis extending therethrough and includes a base having an upper end and a lower end, an upwardly-extending closure member appended to the upper end of the base and formed to include a vent aperture closure cap, and a downwardly-extending, elongated, weighted unit depending from the lower end of the base. The vent aperture closure cap is sized and shaped to close the vent path formed in the housing when the float valve lies in its closed position.
The lower end of the base has a first downwardly-presented surface of a predetermined cross-sectional area that is positioned on the float valve to lie at a position coinciding with the fluid level in the fuel tank when the vent aperture closure cap moves to a position closing the vent aperture in the fuel tank. The weighted unit has a second downwardly-presented surface positioned to lie in axially spaced-apart relation to the first downwardly-presented surface. The second downwardly presented surface has a cross-sectional area that is less than the predetermined cross-sectional area of the first downwardly-presented surface.
The base and the weighted unit are sized, shaped, weighted, and located to cause the float valve to "snap" (e.g. move quickly) to its closed position after rising fuel in the fuel tank rises past the weighted unit and engages the first downwardly-presented surface on the lower end of the base. A float valve in accordance with the present invention is designed to sense and react to predetermined closing conditions quickly yet reduce the incidence of premature closure.
In one embodiment, the float valve is a unitary member formed to include a gas-tight hollow chamber containing the weighted member. In other embodiments, the float valve is a two-piece or multi-piece member formed to include a gas-tight hollow chamber containing the weighted member. In yet another embodiment, the float valve is a unitary, solid core member made of a properly weighted material without a separate weighted member. Illustratively, a float valve in accordance with the present invention includes a skirt, ribs, or other means for guiding movement of the float valve in the housing as the level of liquid fuel in the fuel tank rise and falls.
In a presently preferred embodiment, the float valve includes a body and a conical upper lid having an underside coupled to the body. The body of the float valve includes a cylindrical cup-shaped barrier, an annular lip extending radially outwardly from the barrier, and a skirt extending from the lip about the barrier. The skirt in effect forms a float chamber bounded by the barrier, the lip, and the skirt. The presence of the skirt, below the annular lip, must remain transparent to slowly rising fuel.
The skirt of the body remains transparent to the rising fuel due to vents therethrough which displace fuel vapor from the float chamber. The vents permit fluid flow into the flow chamber so that body becomes buoyant and snaps to the closed position only after the fluid has risen substantially close to the pre-selected closure conditions. It is understood that the vents are situated through the skirt adjacent to the lip so that the fluid rises through the float chamber as the skirt of the valve becomes submerged in the surrounding fluid. Only after the fluid has significantly submerged the skirt does it flow from the float chamber and into the vents of the skirt.
Fluid flow into the vents causes the body to behave as a solid cross-sectional structure which prevents further increases in fluid level within the flow chamber. Preferably, the vents are positioned adjacent the lip so that the rising fluid engages the lip. Thus, the combination of the submerged height of the valve with the sudden increase in the cross-sectional area as perceived by the rising fluid causes the valve to "snap" to the closed position. Thus, the float valve has a reduced dependency upon submerged height of the valve which is a source of premature closure, but has an increased sensitivity to predetermined closure conditions. This has the practical effect of making the float valve much more responsive to sudden sharp changes in fuel level such as when a vehicle carrying a fuel tank containing the float valve is driven on very rough road.
In other embodiments of the present invention, the float valve includes an upper float wall, a lower float wall, and a hollow gas-tight region formed between the upper and lower float walls. The lower float wall includes a lip and means for snapping the upper float wall to the closed position upon engagement of rising fluid with the lip. Illustratively, the upper flow wall is integral with the lower float wall.
In addition, the upper float wall includes a cap and the lower float wall includes a body. The body has a barrier coupled to the lip so that the lip extends radially outwardly from the barrier to create a flow chamber defined by the barrier and the lip.
In yet another embodiment of the present invention, a float valve is provided for use in an apparatus for controlling venting of fuel vapor through an aperture in a vehicle fuel tank. The float valve includes an upper float wall and a lower float wall formed to include a hollow barrier having a substantially constant cross-sectional area and a lip extending radially outwardly from the barrier. The lip has a cross-sectional area greater than cross-sectional area of the barrier so that a product of a cross-sectional area of the float valve, submerged height of the float valve, and specific gravity of fluid surrounding the float valve overcomes the weight of the float valve when the rising fluid engages the lip. Thus, the upper float wall moves toward its closed position.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.